Device isolator with reduced parasitic capacitance

ABSTRACT

Isolator structures for an integrated circuit with reduced effective parasitic capacitance. Disclosed embodiments include methods of forming an integrated circuit including an isolator structure. The isolator structure includes parallel conductive elements forming a capacitor or inductive transformer, overlying a semiconductor structure including a well region of a first conductivity type formed within an tank region of a second conductivity type. The tank region is surrounded by doped regions and a buried doped layer of the first conductivity type, forming a plurality of diodes in series to the substrate. The junction capacitances of the series diodes have the effect of reducing the parasitic capacitance apparent at the isolator

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Nonprovisional PatentApplication serial no. 14/680,211, filed Apr. 07, 2015, now issued as USPat. No. 9,806,148, the contents of which is herein incorporated byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

This invention is in the field of integrated circuits. Embodiments aremore specifically directed to isolator structures in integratedcircuits.

As well known in the art, some implementations of modern integratedcircuits require the communication of signals between integratedcircuits that are not referenced to the same ground voltage, either (orboth) in the DC and AC sense. In those implementations, the directcoupling of input/output terminals could result in a significant voltagedifferential between the respective ground levels. In some applications,this ground voltage differential can be as high as hundreds or thousandsof volts, sufficient to damage the integrated circuits and cause systemfailure. Similarly, transient high voltage spikes at one of theintegrated circuits can couple via the connected input/output terminalsto other integrated circuits. For example, a typical voltage spike at ahigh voltage motor can couple from a motor controller device at themotor to an integrated circuit at a human interface device (e.g.,keypad); such spikes can cause damage and, in the case of a humaninterface device, can affect the human user.

Isolator structures are commonly implemented into integrated circuitsintended for these applications, with those structures deployed at theinput/output terminals. These isolator structures are typically in theform of capacitors or inductors. For the example of an isolatorconstructed as a capacitor, the capacitor is inserted in series betweenthe terminal or pad and the internal circuit. The goal of these isolatorstructures 5 is to absorb the voltage differential between ground levels(or the transient spike) with minimum attenuation of signal information.

FIG. 1a illustrates, in cross-section, the construction of aconventional isolator structure in the form of high voltage capacitor 7deployed in an integrated circuit. In this arrangement, capacitor 7 is aparallel-plate capacitor in which upper plate 8 a and lower plate 8 bare formed in separate metal levels. In this example, capacitor 7 isdeployed directly at an external terminal (e.g., an input) of theintegrated circuit, as evident by wire bond 5 attached to the bond pad(exposed through protective overcoat 9 and top dielectric layer 10 h)that serves as exposed upper plate 8 a. In this conventional example,integrated circuit 4 is fabricated to have seven levels of metalconductors, with lower plate 8 b formed in the second metal level andupper plate 8 a formed in the seventh (topmost) metal level.Accordingly, the intervening dielectric between upper and lower plates 8a, 8 b includes five layers 10 c through 10 g of interlevel dielectricmaterial (e.g., silicon dioxide); two interlevel dielectric layers 10 a,10 b underlie lower plate 8 b, separating it from substrate 11 andisolation dielectric structure 12. Each of interlevel dielectric layers10 serves to insulate adjacent levels of metallization in the verticaldirection in the cross-section of FIG. 1 a. As such, interleveldielectric layers 10 c through 10 g between upper and lower plates 8 a,8 b are formed between the formation and patterned etch of each of theintervening metal levels that form conductors elsewhere in theintegrated circuit. The relatively large cumulative thickness ofinterlevel dielectric layers 10 c through 10 g results in capacitor 7being capable of withstanding and absorbing relatively high voltages.

In addition to capacitor 7 formed between upper and lower plates 8 a, 8b, a parasitic capacitor is defined by the structure shown in FIG. 1 a.Specifically, parasitic parallel-plate capacitor 7 p is present betweenlower plate 8 a and substrate 11 disposed under lower plate 8 b, withthe capacitor dielectric formed of interlevel dielectric layers 10 a, 10b, and isolation oxide structure 12 beneath interlevel dielectric layer10 a. Isolation oxide structure 12 is a conventional dielectricstructure formed into substrate 11, typically with the purpose ofisolating adjacent transistors formed at the surface of substrate 11from one another; isolation oxide structure 12 may be formed by thermaloxidation (e.g., the well-known LOCOS process) or as shallow trenchisolation. Because the cumulative thickness of interlevel dielectriclayers 10 a, 10 b, and isolation oxide structure 12 can be substantiallyless than that of interlevel dielectric layers 10 c through 10 g,parasitic capacitor 7 p can present a significantly larger capacitancethan does high voltage capacitor 7.

The electrical effect of parasitic capacitor 7 p is illustrated in FIG.1 b. High voltage capacitor 7 couples terminal 5 of the integratedcircuit (i.e., wire bond 5 of FIG. 1a ) to internal node 13, which istypically coupled to the internal functional circuitry of the integratedcircuit. However, parasitic capacitor 7 p also couples input 5 to afixed voltage level, for example the substrate voltage Vsub at substrate11 of the integrated circuit (e.g., a ground level), which can causeattenuation of the signal level received at input 5 from that reachinginternal node 13. In an example in which high voltage capacitor 7 has acumulative dielectric thickness (i.e., of interlevel dielectric layers10 c through 10 g cumulatively) of 12.7 μm between upper and lowerplates 8 a, 8 b, and parasitic capacitor 7 p has a cumulative dielectricthickness (i.e., interlevel dielectric layers 10 a and 10 b andisolation dielectric structure 12 cumulatively) of 2.8 μm between lowerplate 8 b and substrate 11, parasitic capacitor 7 p may present acapacitance more than ten times that of high voltage capacitor 7 (e.g.,400 fF vs. 30 fF). Fundamental circuit analysis shows that parasiticcapacitor 7 p results in the signal level at internal node 13 being onlyabout 10 percent of the magnitude of that received at input 5.

By way of further background, another conventional isolator structure isconstructed similarly as capacitor 7 of FIG. 1 a, but includes a dopedwell underlying the bottom plate. With reference to the structure ofFIG. 1 a, this structure would have such a doped well in place ofisolation dielectric structure 12 (albeit at a shallower depth), withthe doping of that well opposite to that of substrate 11 (e.g., ann-well formed into p-type substrate 11).

In either of these conventional structures, conventional approaches toreducing the capacitance of parasitic capacitor 7 p have beenproblematic. For example, forming lower plate 8 b in a higher level ofmetal would increase the dielectric thickness between lower plate 8 band substrate 11, reducing its capacitance. However, this would alsohave the effect of reducing the dielectric thickness between lower plate8 b and upper plate 8 a, as the dielectric of capacitor 7 would bethinner (i.e., fewer interlevel dielectric layers 10 between theplates). This reduced dielectric thickness would, in turn, reduce thehigh voltage isolation capability of capacitor 7. Another approach wouldbe to form both lower plate 8 b and upper plate 8 a in higher levels ofmetal, to increase the dielectric thickness between lower plate 8 b andsubstrate 11 while maintaining the same dielectric thickness forcapacitor 7. However, as evident from FIG. 1 a, upper plate 8 a mayalready be constructed in the highest metal level in the integratedcircuit; accordingly, this approach could require increasing the numberof metal conductor levels from what it otherwise would be, whichincreases the manufacturing cost of the integrated circuit.

As mentioned above, integrated inductors are also used as isolatorstructures, for example in the form of an isolating transformer.Conventional inductive isolator structures are similar to that shown inFIG. 1 a, except that, instead of a parallel plate structure, the twometal conductor levels are patterned as a pair of overlying coils ofsufficient length to define the desired inductance and coupling to oneanother. However, a parasitic capacitance similar to that shown asparasitic capacitor 7 p would be presented between the lower coil andthe underlying substrate. This parasitic capacitor can similarlyattenuate the signal magnitude communicated through the isolatorstructure, as discussed above.

BRIEF SUMMARY OF THE INVENTION

Disclosed embodiments provide an isolator structure with reducedparasitic capacitance.

Disclosed embodiments provide such an isolator structure that does notreduce high voltage isolation capability.

Disclosed embodiments provide such an isolator structure that may bereadily implemented without requiring additional metal levels.

Disclosed embodiments provide a method of fabricating such an isolatorstructure in existing manufacturing process flows.

Disclosed embodiments provide an electronic system including anintegrated circuit having such an isolator structure at an externalterminal connected to another integrated circuit.

Other objects and advantages of the disclosed embodiments will beapparent to those of ordinary skill in the art having reference to thefollowing specification together with its drawings.

According to certain embodiments, an isolator structure is formed near asemiconducting surface of a substrate, in a pair of patterned metalconductor elements overlying one another above the surface, separatedfrom one another by dielectric material and with the lower of theelements separated from the surface by dielectric material. The lowerelement overlies a portion of the substrate of a first conductivity typethat is surrounded by doped portions of a second conductivity type, andthat overlies a buried doped portion of the second conductivity type.The surrounding doped portions and the buried doped portion arephysically in contact on another, isolating the first conductivity typeportion underlying the lower element.

According to other embodiments, the doped portions are electricallyconnected to receive bias voltages so as to reverse bias the p-njunctions among the doped regions.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1a is a cross-sectional view of a portion of a conventionalisolator capacitor structure.

FIG. 1b is an electrical diagram, in schematic form, of the equivalentcircuit of the conventional structure of FIG. 1 a.

FIG. 2 is an electrical diagram, in block form, of an electronic systemconstructed according to disclosed embodiments.

FIG. 3a is a cross-sectional view of a portion of an isolator structureconstructed according to disclosed embodiments.

FIG. 3b is a plan view of the isolator structure of FIG. 3 a, as acapacitor.

FIG. 3c is an electrical diagram, in schematic form, of the equivalentcircuit of the isolator structure of FIGS. 3a and 3 b.

FIG. 3d is a perspective view of the isolator structure of FIG. 3 a, asan inductive transformer.

FIG. 4 is a flow diagram illustrating a method of fabricating anisolator structure according to disclosed embodiments.

FIG. 5 is a cross-sectional view of a portion of an isolator structureconstructed according to another disclosed embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The one or more embodiments described in this specification areimplemented into isolator structures in an integrated circuit, as it iscontemplated that such implementation is particularly advantageous inthat context. However, it is also contemplated that concepts of thisinvention may be beneficially applied to in other applications.Accordingly, it is to be understood that the following description isprovided by way of example only, and is not intended to limit the truescope of this invention as claimed.

As discussed above in connection with the Background of the Invention,some electronic systems are implemented with separate integratedcircuits that are not referenced to the same ground voltage, but whichmust still communicate with one another. Interconnection of theseintegrated circuits to one another, for example by connectinginput/output terminals to one another for the communication of signals,will also couple the voltage differential between the respective groundlevels. As noted above, this ground voltage differential can besufficiently high as to damage the integrated circuits, cause systemfailure, and, in the case of transient high voltage spikes, can becoupled further downstream, including to a human user. As such, isolatorstructures are commonly implemented into integrated circuits intendedfor these applications, with those structures deployed at theinput/output terminals.

FIG. 2 illustrates a portion of an electronic system includingintegrated circuits 14, 16 that are connected to one another. In thisexample, integrated circuit 14 serves as a transmitter of signals tointegrated circuit 16, which is thus the receiver. In this example,integrated circuit 14 includes functional circuitry, such as pulse-widthmodulator 15 a and linear function 15 b, each of which are coupled viarespective output drivers 17 a, 17 b to output terminals 18. Theseoutput terminals 18 of integrated circuit 14 are directly connected toinput terminals of integrated circuit 16, at which isolator structures20 are deployed according to these embodiments. While not shown, similarisolator structures may be implemented at output terminals 18 ofintegrated circuit 14. As evident from FIG. 2, isolator structures 20communicate the received signals to input amplifiers 21 a, 21 b, whichin turn are coupled to CPU 22 in this example. In this example, outputdrivers 17 a, 17 b of integrated circuit 14 are referenced to one groundlevel GND14, while input amplifiers 21 a, 21 b of integrated circuit 16are referenced to a different ground level GND16. As discussed above,these ground levels GND14, GND16 may be at significantly differentvoltages from one another, relative to some external reference, and assuch that difference will couple between integrated circuits 14, 16.According to these embodiments, isolator structures 20 may beconstructed as capacitors or inductive transformers that are sufficientto absorb the differential between ground levels and also any transientvoltage spikes from integrated circuit 14, while minimizing attenuationof the communicated signal.

FIGS. 3a and 3b illustrate, in cross-section and plan views,respectively, the construction of isolator structure 20 in integratedcircuit 16 according to an embodiment. This example will be describedfor the example in which isolator structure 20 is in the form of aparallel-plate capacitor formed by upper plate 28 a and lower plate 28b. Alternatively, isolator structure 20 may be formed as an inductivetransformer having a similar cross-section to that of FIG. 3 a, butdiffering in the shape of the parallel elements as will be discussedbelow. As shown in FIG. 3 a, wire bond 35 is attached to upper plate 28a; as such, upper plate 28 a serves as a bond pad, connected directly toan external terminal of integrated circuit 16, and as such is formed inan upper if not topmost metal conductor level. Wire bond 35 extendsthrough an opening etched in protective overcoat 29 and interleveldielectric layer 30 h, in the conventional manner. While a ball bond isillustrated in FIG. 3 a, it is of course contemplated that externalcontact to upper plate 28 a may be made according to any one of a numberof techniques, depending on the packaging technology, including stitchbonding, beam lead contact, solder bumps, and the like.

Alternatively, isolator structure 20 may be indirectly coupled to theexternal terminal, for example by either of upper and lower plates 28 a,28 b being connected by way of an intermediate conductor to the bond padreceiving bond wire 35. However, it is contemplated that isolatorstructure 20 most efficiently isolates integrated circuit 16 by itsimplementation directly at the bond pad or otherwise directly in contactwith the external terminal.

According to this embodiment, integrated circuit 16 is constructed withmultiple metal conductor levels, with upper plate 28 a in the topmost orat least an upper one of those levels, as mentioned above. Conversely,lower plate 28 b is implemented in a lower metal conductor level, suchas the first or second metal level, as shown in FIG. 3 a. Alternatively,lower plate 28 b may be formed of polycrystalline silicon, such as usedto realize transistor gate elements; as known in the art, polysiliconlevels are typically formed beneath all metal conductor levels,typically as the first conductive layer above the semiconductor surfaceof the substrate. In the example of FIG. 3 a, in which several metalconductor levels are used within integrated circuit 16, severalinterlevel dielectric layers 30 b through 30 g vertically separate lowerplate 28 b from upper plate 28 a. These interlevel dielectric layers 30b through 30 g constitute the capacitor dielectric of isolator structure20 in this embodiment.

In this embodiment, lower plate 28 a overlies n-type well 32 w,separated therefrom by interlevel dielectric layer 30 a and anyremaining gate dielectric (not shown). As known in the art forcomplementary metal-oxide-semiconductor (CMOS) integrated circuits,n-well region 32 w is a typical n-doped region formed into the surfaceof p-type substrate or a p-type epitaxial layer, as the case may be. Asshown in FIG. 3 a, n-well region 32 c is similarly disposed at thesurface, separated from and surrounding n-well 32 w as evident in FIG. 3b. In this example, n-type well 32 w is at the surface of p-type tankregion 31 t, which is formed of p-type epitaxial single-crystal silicon.P-type tank region 31 t overlies n-type buried layer 36, which is formedwithin p-type substrate 31 s. N-type buried layer 36 verticallyseparates p-type substrate 31 s from p-type tank region 31 t. As evidentfrom FIG. 3 a, n-type buried layer 36 laterally extends beyond thelateral dimensions of n-well 32 w, and is in contact with n-type buriedisolation region 34.

Buried isolation region 34 is disposed beneath but in contact over itslength with n-well region 32 c. As mentioned above, n-well region 32 csurrounds n-well 32 w; similarly, buried isolation region 34 surroundsn-well 32 w, and is in contact over its length with n-type buried layer36 over its length. Accordingly, the combination of n-well region 32 c,buried isolation region 34, and n-type buried layer 36 surrounds anddefines p-type tank region 31 t as a p-type region that is electricallyisolated from p-type substrate 31 s within the silicon structure (i.e.,the bulk). FIG. 3b illustrates that n-well region 32 w extends on allsides of lower plate 28 b, p-type tank region 31 t surrounds n-wellregion 32 w, and n-well region 32 c surrounds p-type tank region 31 t.As shown in FIGS. 3a and 3 b, p-type substrate 31 s extends to thesurface of the single-crystal silicon at locations outside of n-wellregion 32 c, relative to lower plate 28 b.

According to this embodiment, the metallurgical junction (i.e., p-njunction) between n-well region 32 w and p-type tank region 31 t iselectrically reflected in a diode, shown in FIG. 3a as diode D1.Similarly, the p-n junction between p-type tank region 31 t and n-typeburied layer 36 (and buried isolation region 34 and n-type well region32 c, for that matter) establishes diode D2, and the p-n junctionbetween n-type buried layer 26 and p-type substrate 31 s establishes D3.As will be described in further detail below, these p-n junctions (i.e.,diodes) each present a significant junction capacitance to thestructure, and have the effect of reducing the effective parasiticcapacitance presented to isolator structure 20.

In this embodiment, metal conductors 40 are provided to apply biasvoltages to at least some of the doped regions of isolator structure 20,specifically to reverse bias the p-n junctions. In the example shown inFIG. 3 a, metal conductors 40 w contact p+ doped regions 38 s that aredisposed at one or more surface locations of substrate 31 s. P+ dopedregions 38 s are more heavily-doped regions formed into the surface, forexample having dopant concentrations similar to p-type source/drainregions in PMOS transistors formed elsewhere in integrated circuit 16,that assist the forming of ohmic contact between overlying metalconductors 40 s and substrate 31 s. These metal conductors 40 w includeboth the metal lines running in one of the metal conductor levels ofintegrated circuit 16 (the second level from the topmost in this case),metal lines running into and out of the page in the view of FIG. 3 a,and the conductive plugs formed in vias through the various interleveldielectric layers 30 between that metal conductor level and the surfaceof p+ doped region 38 s. Similarly, metal conductors 40 c contact one ormore of surrounding n-well regions 32 c (which may similarly have a moreheavily-doped n-type portion at the surface to provide ohmic contact,similarly to p+ doped regions 38 s, depending on the dopantconcentration of n-well region 32 c), including plugs extending to oneof the metal conductor levels and the metal line at that level.Similarly, metal conductor 40 t contacts p+ doped regions 38 t at one ormore surface locations of tank region 31 t, and metal conductors 40 wcontacts (via heavily doped n-type contact regions, if desired) at oneor more surface locations of n-well region 32 w.

FIG. 3c is an electrical schematic illustrating the equivalent circuitof isolator structure 20 of FIGS. 3a and 3 b. As shown in FIG. 3 c,upper plate 28 a and lower plate 28 b form capacitor 27, with upperplate 28 a shown as the node connected to bond wire 35 (and thus to anexternal terminal of integrated circuit 16), and lower plate 28 b as thenode connected to one of input amplifiers 21 a, 21 b (FIG. 2). Parasiticcapacitor 27 p is formed between lower plate 28 b and n-well region 32w.

As noted above, the construction of isolator structure 20 according tothis embodiment adds p-n junctions and corresponding junctioncapacitances, to the structure below lower plate 28 b. As shown in theschematic of FIG. 3 c, the p-n junction between n-well region 32 w andtank region 31 t presents diode D1 and its junction capacitance CD1. Thep-n junction between tank region 31 t and the combination of n-wellregion 32 c, buried isolation regions 34, and n-type buried layer 36presents diode D2 and its junction capacitance CD2, and the p-n junctionbetween n-well regions 32 c, buried isolation regions 34, and n-typeburied layer 36, on one hand, and p-type substrate 31 s on the otherhand, presents diode D3 and its junction capacitance CD3. Thecapacitance values of these junction capacitances CD1, CD2, CD3 willdepend on the dopant concentrations of the contacting regions at themetallurgical junction, as well as the area of that junction.

These junction capacitances CD1, CD2, CD3 are effectively in series withparasitic capacitor 27 p between capacitor 27 of isolator structure 20and substrate 31 s. Applying fundamental circuit analysis to thisstructure, the effect of these series capacitances CD1, CD2, CD3 is toreduce the parasitic capacitance at lower plate 28 b from that whichwould be presented by parasitic capacitor 27 p alone, for example by onthe order of 20% in one implementation.

As noted above relative to FIG. 3 a, conductors 40 allow the applicationof bias voltages to the various regions of isolator structure 20, inparticular bias voltages that establish a reverse bias condition acrosseach of the p-n junctions. As is well-known in the art, application of areverse bias voltage to a p-n junction increases the width of thespace-charge region at the junction, and thus increases the effectivejunction capacitance presented by the junction. For example, applicationof a voltage Vbias+ to n-well region 32 w and n-type buried layer 36(via regions 32 c, 34), and a voltage Vbias− to tank region 31 t andsubstrate 31 s, where voltage Vbias+ is greater than voltage Vbias−,will increase the capacitance values of junction capacitances CD1, CD2,CD3. This increase in those series capacitances CD1, CD2, CD3, in thecircuit arrangement of FIG. 3 c, will further reduce the effectiveparasitic capacitance at lower plate 28 b. For example, application ofbias voltage Vbias+ of about 1.8 volts (relative to bias voltage Vbias−at ground) has been observed to reduce the overall parasitic capacitanceby on the order of 25% from that of the unbiased condition, or on theorder of 40% overall. While bias voltages Vbias+, Vbias− need not beapplied to isolator structure 20 in order to obtain significantreduction in the parasitic capacitance presented by parasitic capacitor27 p, and as such is an optional feature in these embodiments, theapplication of these bias voltages Vbias+, Vbias− further reduces theeffective parasitic capacitance.

As shown in the electrical schematic of FIG. 3 c, the bias voltagesVbias+, Vbias− are applied to each instance of the respective conductorsvia a resistor 42. In this embodiment, resistors 42 are relatively largeresistors, with resistances on the order of tens of thousands of ohms(e.g., 30 kΩ). These resistors 42 ensure that the voltage sourcesapplying bias voltages Vbias+, Vbias− do not establish a low impedancepath that would effectively short circuit diodes D1, D2, D3. While notshown in FIGS. 3 a, 3 b, it is contemplated that resistors 42 may beconstructed in the conventional manner for high value resistors, such asby way of lightly-doped polysilicon structures or doped regions formedinto the bulk.

As mentioned above, the isolator structure according to theseembodiments may be realized as an inductive transformer rather than as acapacitor as shown in FIGS. 3a and 3 b. According to one embodiment, asshown in a perspective view in FIG. 3d , an inductive transformerimplementation of isolator structure 20′ is constructed to include upperand lower coils 28 a′, 28 b′ in different metal conductor levels,separated from one another by interlevel dielectric layers 30 similarlyas shown in FIG. 3 a, but with each in the shape of a coil to form aninductor. Because of its coil shape, upper coil 28 a′ will typically beconnected to a bond pad to receive wire bond 35, rather than itselfserve as the bond pad. The underlying structure of isolator structure20′ corresponds to that described above relative to the capacitiveimplementation of FIGS. 3a through 3 c. And similarly, while parasiticcapacitance 27 p is also presented in this isolator structure 20′between lower coil 28 b′ and n-well 32, the construction of isolatorstructure 20′ in this manner to include series diodes D1 through D3 andthe corresponding junction capacitances CD1 through CD3 reduces theeffect of that parasitic capacitance, and thus reduces the attenuationon the received signal.

While the above description refers to certain of the doped regions inisolator structure 20 as n-type and others as p-type, it is of coursecontemplated that the structure may alternatively be constructed usingdoped regions and a substrate of opposite conductivity types (i.e., then-type and p-type regions shown in FIG. 3a instead being p-type andn-type, respectively). In that alternative construction, of course, therelative polarity of the applied bias voltages Vbias+, Vbias− will bereversed, so as to maintain reverse biased junctions by ensuring thatthe higher voltage is applied to n-type regions and the lower voltage isapplied to p-type regions.

It is contemplated that the isolator structure, whether as a capacitoror as a an inductive transformer, may be fabricated according to theseembodiments using process flows already in place for modern integratedcircuits. In particular, it is contemplated that such isolatorstructures may be implemented into a conventional CMOS process flowutilizing buried layers merely by way of changes to photomasks, andwithout significant change to the manufacturing process. Accordingly, itis contemplated that those skilled in the art having reference to thisspecification will be readily able to derive a workable manufacturingprocess flow, without undue experimentation. For example, commonlyassigned U.S. Patent Application Publication No. US 2010/0032769 A1,published Feb. 11, 2010 and incorporated herein by this reference,describes a fabrication process suitable for the construction ofintegrated circuits including isolator structures according to theembodiments described above.

Referring now to FIG. 4 (together with FIG. 3a ), an example of aprocess of forming isolator structure 20 in an integrated circuitaccording to an embodiment, generally following the approach describedin the above-incorporated Patent Application Publication, will bedescribed. According to this embodiment, the fabrication process beginswith process 50, in which masked ion implantation of n-type dopant isperformed to selected locations of a p-type substrate to define thelocation of n-type buried layer 36. As described in theabove-incorporated Patent Application Publication No. US 2010/0032769,implant process 50 may implant n-type dopants such as antimony at a doseand energy sufficient to place the desired amount of dopant to a desireddepth within the substrate. In this embodiment, epitaxial silicon willbe grown at the surface; as such, process 50 may be performed by aconventional ion implantation at the surface of the substrate. Process50 may also include a high temperature anneal to diffuse the implanteddopant to at least a partial extent in the substrate. Similarly, implantprocess 52 involves the masked ion implantation of n-type dopant, suchas phosphorous and possibly including arsenic, at locations of thesurface of the substrate at which n-type buried isolation region 34 isto be implanted. In this embodiment, as indicated in FIG. 3 b, thelocations implanted in process 52 include a boundary region encirclingthe eventual location of the capacitor or transformer to serve as theisolator element.

In process 54 according to this embodiment, epitaxial growth of siliconis then performed, to grow a layer of p-type silicon at the surface ofthe p-type substrate. The thickness of this epitaxial layer is intendedto be sufficient to define tank region 31 t of FIG. 3a above n-typeburied layer 36 and n-type buried isolation region 34. As described inthe above-incorporated Patent Application Publication No. US2010/0032769, the temperature and duration of epitaxy process 54 issufficient to diffuse the dopant implanted in process 52 and to furtherdiffuse the dopant implanted in process 50, effectively forming thestructures of n-type buried layer 36 and n-type buried isolation region34 as shown in FIG. 3 a, with p-type epitaxial silicon overlying thoseregions. This diffusion is also contemplated to cause n-type buriedisolation region 34 to grow into and contact n-type buried layer 36, andthus isolate p-type tank region 31 t from substrate 31 s (substrate 31 snow including p-type epitaxial material overlying and in contact withthe original substrate).

In process 56, n-well regions 32 w and 32 c are implanted at selectedlocations into the surface of the structure, which is now the surface ofthe p-type epitaxial silicon formed in process 54. In particular, theimplant of process 56 implants n-type dopant (e.g., phosphorous,arsenic) at a location within tank region 31 t and underlying theeventual location of lower plate 28 b, to form n-well region 32 w, andat locations overlying n-type buried isolation region 34, to form n-wellregion 32 c. The dose and energy of this well implant is selected in theconventional manner to form the well regions appropriate for p-channelMOS transistors elsewhere in the integrated circuit, for example.Process 56 also includes the appropriate high temperature anneal fordiffusing the implanted dopant to the desired depth and profile. Forpurposes of isolator structure 20 of this embodiment, the dose, energy,and anneal conditions of process 56 are selected so that n-well region32 c reach and contact the underlying n-type buried isolation region 34,as shown in FIG. 3 a.

In process 58, a dielectric layer is formed overall, for example bythermal oxidation of the semiconductor surface or by deposition of thedesired dielectric material. Typically, for integrated circuitsincluding MOS transistors, process 58 forms the gate dielectric film.Following process 58, polycrystalline silicon is deposited in thisembodiment, followed by patterning and etching of the polysilicon todefine the desired structures in the integrated circuit, for exampletransistor gate structures. This polysilicon may be doped in situ duringits deposition, or alternatively may be implanted after deposition andetch. For the embodiment in which resistors 42 are incorporated intoisolator structure 20, process 60 may form these resistors 42 inpolysilicon deposited and defined in this process 58, particularly withthat polysilicon being relatively lightly-doped (if at all) so as tohave a high resistance and thus efficiently implement a high resistorvalue (e.g., 30 kΩ).

If desired, lower plate 28 b may be formed in this polysilicon level,rather than in a metal level. While this would increase the parasiticcapacitance in conventional isolator structures, such as that shown inFIG. 1 a, when implemented according to this embodiment, the seriesjunction capacitances CD1, CD2, CD3 at the underlying p-n junctionsserve to reduce that parasitic capacitance. This reduction in theparasitic capacitance may, in some implementations allow the formationof lower plate 28 b in polysilicon rather than in a higher metal level,thus improving the high voltage capability of the isolator or, in somecases, enable the reduction of the number of metal levels requiredwithout affecting the high voltage tolerance.

In process 62, typically following the defining of polysilicon elementsin MOS integrated circuits, n-type and p-type source/drain regions areformed by ion implantation of the appropriate dopant species, followedby high temperature anneal. In this embodiment p+ contact regions 38 s,38 t (FIG. 3a ) are also formed in this source/drain anneal process 62,as are corresponding n+ contact regions in n-well regions 32 w, 32 c ifdesired.

Conductor levels including those used to form lower plate 28 b and upperplate 28 a are the formed by a sequence of processes 64, 66, 68. Inprocess 64, an interlevel dielectric layer 30, such as silicon dioxideor silicon nitride, is deposited by conventional processes to thedesired thickness. In process 66, contact openings (for metal to siliconcontacts) or vias (for metal to metal contacts) are patterned andetched. Deposition of contact plugs as appropriate to fill vias openedin process 66, and deposition of a metal layer are then performed inprocess 68. Process 68 also includes the desired photolithographicpatterning and metal etch to define the metal conductors to be formed inthis particular level of metal. As described above, this integratedcircuit 16 is contemplated to be constructed with multiple metalconductor levels; as such, processes 64, 66, 68 are repeated a number oftimes corresponding to the number of metal levels to be formed. If notpreviously formed in polysilicon, as described above, lower plate 28 amay be formed in one of the lower metal levels formed in this sequenceof processes, 64, 66, 68; upper plate 28 a will be formed in a higherone of those metal levels, to attain the desired high voltage capabilityof the isolator.

Following the formation and patterning of the topmost metal layer in thelast instance of process 68, process 70 is then performed to deposit aprotective overcoat, such as silicon nitride, overall. Process 70 alsoincludes the patterned etch of openings through this protectiveovercoat, such as that etched over upper plate 28 a to allow contact bybond wire 35 as shown in FIG. 3 a. Such other “back end” processing asappropriate to complete, test, and package integrated circuit 16 asdesired for its system implementation such as shown in FIG. 2 may thenbe carried out.

The above embodiments incorporate isolator structure 20 into integratedcircuit 16 in such a manner that a single isolated tank (p-type tankregion 31 t in FIG. 3a ) is formed. However, it is contemplated thatadditional reduction in the parasitic capacitor may be attained byforming the structure so as to have multiple isolated tanks in series,essentially by forming multiple tank regions nested within one anotherso that additional series diodes and junction capacitances are presentin the structure. FIG. 5 illustrates an embodiment of such an isolatorstructure 20′, in which two such nested tanks are implemented as willnow be described.

Isolator structure 20′ of FIG. 5, according to this embodiment, includesmany of the same components as described above relative to isolatorstructure 20 of FIGS. 3a and 3b ; those common elements are indicated inFIG. 5 by the same reference numerals as used in connection with FIGS.3a and 3 b. As such, isolator structure 20′ is realized by aparallel-plate capacitor formed by upper plate 28 a and lower plate 28 bformed in separate conductor levels, separated by dielectric material inone or more interlevel dielectric layers 30 b through 30 g, which serveas the capacitor dielectric. A wire bond (not shown) will be attacheddirectly or indirectly to upper plate 28 a in the manner describedabove, for example through the opening in protective overcoat 29 andinterlevel dielectric layer 30 h shown in FIG. 5. As in the embodimentdescribed above, lower plate 28 a of isolator structure 20′ overliesn-type well 32 w, with an interlevel dielectric layer and perhaps a gatedielectric layer separating lower plate 28 a from well 32 w. P-type tankregion 31 t underlies n-type well region 32 w, isolated on all sides byn-type well region 32 c and n-type buried isolation region, and beneathby n-type buried layer 36. As described above relative to FIG. 3a and asshown in FIG. 5, n-type buried layer 36 laterally extends beyond thelateral dimensions of n-well 32 w, and is in contact with n-type buriedisolation region 34.

According to this embodiment, a second isolated p-type tank region 31 t′surrounds n-well region 32 c, buried isolation region 34, and n-typeburied layer 36. This second p-type tank region 31 t′ is itselfsurrounded by the n-type material of second n-type buried layer 86,buried isolation region 84, and another instance of n-well region 32 c.Buried isolation region 84 extends deeper into the structure than buriedisolation region 34, so as to contact the correspondingly deeper secondn-type buried layer 86 and fully isolate second p-type tank region 31 t′from p-type substrate 31 s. P-type substrate 31 s underlies deepersecond n-type buried layer 86, and extends to the surface outside ofn-type buried isolation region 84 and the second instance of n-wellregion 32 c.

If isolator structure 20′ were viewed from above (i.e. similarly as theplan view of FIG. 3b ), the second p-type tank region 31 t′ wouldsurround the inner n-well region 32 w and would itself be surrounded atthe surface by the second, outer, n-well region 32 c.

According to this embodiment, one additional p-n junction (diode D4 ofFIG. 5) is present between second tank region 31 t′ and second n-typeburied layer 86, and another p-n junction (diode D5) is present betweensecond n-type buried layer 86 and underlying substrate 31 s. Thesediodes D4, D5 are in series with diodes D1 through D3 defined by thestructure of n-well 32 w, first p-type tank region 31 t, and firstn-type buried layer 36 (diode D3 at the p-n junction between n-typeburied layer 36 and second p-type tank region 31 t′ in this case). Asdescribed above, these additional p-n junctions establishing diodes D4and D5 each present a significant junction capacitance to the structure.These capacitances will be in series with the junction capacitancesassociated with diodes D1 through D3, and in series with the parasiticcapacitor between lower plate 28 b and n-well 32 w. As such, theseadditional p-n junctions provided by the additional isolated p-tankregion 31 t′ will further reduce the effective parasitic capacitance ofisolator structure 20′.

If desired, metal conductors 40 may also similarly make contact to thevarious doped regions of isolator structure 20′ to establish reversebias conditions at each of the p-n junctions in the structure. Asdiscussed above, a reverse bias voltage applied to the p-n junctionsestablishing diodes D1 through D5 will increase the junction capacitanceat those locations, further reducing the parasitic capacitance ofisolator structure 20′. In the same manner as discussed above relativeto FIGS. 3a through 3 d, this reverse bias condition can be applied tothe junctions established by n-type buried layer 86 through conductors40 t′ at doped regions 38 t′ within second tank 32 t′, additionalconductors 40 c at n-well region 32 c (and at more heavily doped n-typeregions within those wells, if desired), and conductors 40 s at p-typedoped regions 38 s within surface portions of substrate 31 s, as shownin FIG. 5. Conductors 40 c and 40 w will receive a positive voltage(e.g., a bias voltage Vbias of +1.8 volts) relative to the voltage(e.g., a bias voltage Vbias− at ground level) applied to conductors 40t, 40 t′, 40 s. Also as discussed above, resistors (not shown) arepreferably are provided in series with these conductors 40 and theirrespective voltage sources, to prevent short-circuiting of the junctionsas the bias voltages are applied. Again, while these reverse biasvoltages are not essential to obtain significant reduction of theparasitic capacitance, and as such is an optional feature, this biasfurther reduces the effective parasitic capacitance by increasing thejunction capacitance of diodes D1 through D5.

It is contemplated that the additional n-type buried layer 86 and buriedisolation regions 84 may be formed into the structure by way ofconventional processes such as those described above relative to FIG. 4.For example, one approach to the fabrication of these regions wouldinvolve the repeating of implant processes 50, 52 and epitaxy process 54after a first instance of those processes, as indicated by the dashedline from process 54 to process 50 as shown in FIG. 4. The two instancesof buried isolation region implant process 52 would, in this example, beperformed at the same locations so as to form the extended depth ofburied isolation structure 84. Other processes for forming these nestedisolated tanks are also contemplated.

According to these embodiments, an isolator structure with reducedeffective parasitic capacitance, yet without reducing high voltageisolation performance, is provided. This structure is suitable for useeither as a capacitive isolator or as an inductive isolator such as atransformer, and may be efficiently fabricated using manufacturingprocesses such as may be otherwise used or available for the integratedcircuit into which the isolator structure is formed. It is contemplatedthat isolator structures according to some of these embodiments canenable fabrication of a lower plate or coil element in a lower conductorlayer, including polysilicon levels, with tolerable parasiticcapacitance as a result of these isolator construction, which can enableimproved high voltage isolation performance.

While one or more embodiments have been described in this specification,it is of course contemplated that modifications of, and alternatives to,these embodiments, such modifications and alternatives capable ofobtaining one or more the advantages and benefits of this invention,will be apparent to those of ordinary skill in the art having referenceto this specification and its drawings. It is contemplated that suchmodifications and alternatives are within the scope of this invention assubsequently claimed herein.

What is claimed is:
 1. A method of forming an integrated circuit,comprising: forming a buried doped layer of a first conductivity type ina semiconductor substrate, the semiconductor substrate having a secondconductivity type; forming a buried isolation region of the firstconductivity type in the semiconductor substrate so as to contact theburied doped layer; forming a plurality of well regions of the firstconductivity type at a surface of the semiconductor substrate, a firstone of the well regions formed into a tank region of the secondconductivity type disposed above the buried doped layer and surroundedby the buried isolation region, and a second one of the well regionsoverlying and in contact with the buried isolation region; then forminga first dielectric layer over the surface of the semiconductorsubstrate; forming a first conductor element overlying the firstdielectric layer at a location overlying the first well region; forminga second dielectric layer over the surface of the semiconductorsubstrate; and forming a second conductor element overlying the seconddielectric layer at a location overlying the first conductor element. 2.The method of claim 1, wherein the step of forming the first conductorelement comprises: depositing a first conductive layer over the firstdielectric layer; and patterning the first conductive layer to define afirst capacitor plate at the location overlying the first well region;and wherein the step of forming the second conductor element comprises:depositing a second conductive layer over the second dielectric layer;and patterning the second conductive layer to define a second capacitorplate at the location overlying the first capacitor plate.
 3. The methodof claim 1, wherein the step of forming the first conductor elementcomprises: depositing a first conductive layer over the first dielectriclayer; and patterning the first conductive layer to define a firstinductor coil at the location overlying the first well region; andwherein the step of forming the second conductor element comprises:depositing a second conductive layer over the second dielectric layer;and patterning the second conductive layer to define a second inductorcoil at the location overlying the first capacitor plate.
 4. The methodof claim 1, further comprising: forming first and second contact regionsof the second conductivity type at the surface of the semiconductorsubstrate, the first contact region disposed at a location of thesurface of the semiconductor substrate between the first and second wellregions and in contact with the tank region, and the second contactregion disposed at a location of the surface of the semiconductorsubstrate outside of the second well region relative to the tank regionand in contact with the surface of the semiconductor substrate.
 5. Themethod of claim 4, further comprising: forming a plurality of metalconductors overlying the surface of the semiconductor substrate,respective ones of the plurality of metal conductors in ohmic contactwith the first contact region, the second contact region, the first wellregion, and the second well region.
 6. The method of claim 5, furthercomprising: forming a plurality of resistors near the surface of thesemiconductor substrate, respective ones of the resistors in contactwith respective ones of the plurality of metal conductors.
 7. The methodof claim 5, wherein the steps of forming the buried doped layer andforming the buried isolation region each comprise: implanting dopant ofthe first conductivity type into the surface of the semiconductorsubstrate; and further comprising: after the implanting steps,epitaxially growing silicon of the second conductivity type over theimplanted regions to form the tank region overlying the buried dopedlayer.
 8. The method of claim 1, further comprising: then forming athird dielectric layer overlying the second conductor element; etchingan opening in the third dielectric layer to expose a portion of thesecond conductor element; and contacting the exposed portion of thesecond conductor element with a conductor in contact with an externalterminal of the integrated circuit.
 9. The method of claim 1, whereinthe step of forming a buried doped layer comprises: forming first andsecond buried doped layers of the first conductivity type into thesemiconductor substrate, the first buried doped layer underlying thesecond doped layer and separated therefrom by semiconductor material ofthe second conductivity type; wherein the step of forming a buriedisolation region comprises: forming a first buried isolation region ofthe first conductivity type into the substrate so as to contact thefirst buried doped layer; and forming a second buried isolation regionof the first conductivity type into the substrate so as to contact thesecond buried doped layer; wherein the second one of the well regions isoverlying and in contact with the first buried isolation region, andwherein the step of forming a plurality of well regions also forms athird one of the well regions overlying and in contact with the secondburied isolation region.
 10. A method of forming an integrated circuit,comprising: forming a buried doped layer of a first conductivity typeinto a semiconductor substrate, the semiconductor substrate having asecond conductivity type; forming a buried isolation region of the firstconductivity type into the semiconductor substrate so as to contact theburied doped layer; forming a plurality of well regions of the firstconductivity type at a surface of the semiconductor substrate, a firstone of the well regions formed into a tank region of the secondconductivity type disposed above the buried doped layer and surroundedby the buried isolation region, and a second one of the well regionsoverlying and in contact with the buried isolation region; then forminga first dielectric layer over the surface of the semiconductorsubstrate; forming a first conductor element overlying the firstdielectric layer at a location overlying the first well region; forminga second dielectric layer over the surface of the semiconductorsubstrate; and forming a second conductor element overlying the seconddielectric layer at a location overlying the first conductor element;then forming a third dielectric layer overlying the second conductorelement; etching an opening in the third dielectric layer to expose aportion of the second conductor element; and contacting the exposedportion of the second conductor element with a conductor in contact withan external terminal of the integrated circuit.
 11. The method of claim10, wherein the step of forming the first conductor element comprises:depositing a first conductive layer over the first dielectric layer; andpatterning the first conductive layer to define a first capacitor plateat the location overlying the first well region; and wherein the step offorming the second conductor element comprises: depositing a secondconductive layer over the second dielectric layer; and patterning thesecond conductive layer to define a second capacitor plate at thelocation overlying the first capacitor plate.
 12. The method of claim10, wherein the step of forming the first conductor element comprises:depositing a first conductive layer over the first dielectric layer; andpatterning the first conductive layer to define a first inductor coil atthe location overlying the first well region; and wherein the step offorming the second conductor element comprises: depositing a secondconductive layer over the second dielectric layer; and patterning thesecond conductive layer to define a second inductor coil at the locationoverlying the first capacitor plate.
 13. The method of claim 10, furthercomprising: forming first and second contact regions of the secondconductivity type at the surface of the semiconductor substrate, thefirst contact region disposed at a location of the surface of thesemiconductor substrate between the first and second well regions and incontact with the tank region, and the second contact region disposed ata location of the surface of the semiconductor substrate outside of thesecond well region relative to the tank region and in contact with thesubstrate.
 14. The method of claim 13, further comprising: forming aplurality of metal conductors overlying the surface of the semiconductorsubstrate, respective ones of the plurality of metal conductors in ohmiccontact with the first contact region, the second contact region, thefirst well region, and the second well region.
 15. The method of claim14, further comprising: forming a plurality of resistors near thesurface of the semiconductor substrate, respective ones of the resistorsin contact with respective ones of the plurality of metal conductors.16. The method of claim 14, wherein the steps of forming the burieddoped layer and forming the buried isolation region each comprise:implanting dopant of the first conductivity type into the surface of thesemiconductor substrate; and further comprising: after the implantingsteps, epitaxially growing silicon of the second conductivity type overthe implanted regions to form the tank region overlying the buried dopedlayer.